There is known a water-droplet detecting apparatus that detects a water droplet such as a raindrop adhered to a lens of a camera attached to a vehicle. For example, the water-droplet detecting apparatus detects the adhesion of the water droplet on the basis of a gradient of each pixel of a captured image (see Japanese Laid-open Patent Publication No. 2010-014494, for example).
However, the conventional technology is needed to detect the gradients of all of the pixels of the captured image, and thus there exists a fear that a processing load is large. Moreover, narrowing down a region to be detected from a whole of the captured image is difficult, and thus the water droplet is not always detected with high accuracy.